This proposal aims to delineate cellular and molecular mechanisms responsible for the selective survival of medium aspiny neurons, and death of neighboring neurons in the striatum of Huntington's Disease (HD). Surviving medium aspiny neurons in HD brain display a unique biochemical phenotype that is reproduced in cell culture models obtained by applications of the NMDA receptor (NMDAR) agonist QUIN. These neurons colocalize somatostatin (SS), neuropeptide Y (NPY), and the enzyme nitric oxide synthase (NOS) which catalyzes the synthesis of nitric oxide (NO). We have shown that incubation of cultured neurons with QUIN selectively upregulates SS (but not NPY) peptide and MRNA levels. We have also found that in cultured tumor cells SS mediates apoptotic cell death by interacting with cell cycle regulatory protein kinases. Considering the unique molecular alterations of the medium aspiny neuron, we postulate that the interplay between NMDAR signalling, SS and NOS/NO provide the substrate for the phenomenon of selective cell survival and cell death. Our longterm goal is to determine (i) what influences survival of the SS/NPY/NOS cell. (ii) whether this cell is involved in mediating adjacent neurotoxicity and (iii) the mechanism of upregulation of SS gene expression in this cell. We hypothesize that augmented SS function is a critical determinant of SS/NPY/NOS cell survival, and that SS acting on neighboring cells in concert with NMDA and NO potentiates the neurotoxic process. Using as models normal rat brain and QUIN exposed striatal cultures, we will determine whether survival of the SS/NPY/NOS neuron is due to lack of NMDAR by in situ hybridization studies of NMDAR gene expression. In situ hybridization will also be used to map the expression of SS receptor subtype mRNAs to delineate the intrastriatal targets of SS action. The effects of functional ablation of SS with antisense oligonucleotides and by in vitro immunoneutralization with anti SS and anti SS receptor antibodies on the capacity of the SS/NPY/NOS neuron to survive NMDA toxicity will be examined. The functional status of NOS in QUIN treated SS/NPY/NOS cell will be determined by Northern analysis of NOS mRNA and by testing the effect of NOS inhibitors with or without functional SS ablation on cell survival and surrounding toxicity. The role of apoptosis in NMDA induced neurotoxicity and the participation of SS in this process will be studied by flow cytometry of QUIN treated striatal cultures and in presynchronized PC12 cells stably transfected with SS receptor cDNAS. Finally, the role of the NO/cGMP pathway in mediating NMDAR induced SS gene upregulation will be investigated with appropriate agonists and antagonists. These studies will provide new insights into the molecular mechanisms underlying the unique pattern of neuronal loss and preservation in HD. Furthermore, the results could provide new insights into the problem of neurodegeneration in general and offer clues for potential therapeutic interventions in HD.